The use of fresh water ponds for watering stock has been a benefit to the ranchers in this country. However the breeding of mosquitos that carry the West Nile Virus in such ponds has also been a serious problem. Mosquitos and other biting insects can be a nuisance interfering with work and leisure time, and can cause the reduced production of milk and meat in livestock. Some mosquito species are capable of transmitting diseases to humans and/or animals including malaria, yellow fever, filarias, encephalitis, and West Nile virus.
The use of oil films on water surfaces to kill mosquito larvae is also well established and has been used for many years. Because of their adverse environmental impact petroleum fuel products such as diesel fuel, used initially as thin film larvicides have now been replaced by less toxic organic oil Examples. Modern commercial mosquito control products include highly refined mineral oils and organic acid surfactants that form films when applied to the water surface.
Fatty acid esters have been found to be a desirable choice since they are environmentally safe, have a relatively low viscosity, and can form continuous films on water surfaces that act as larvicides. The problem with the application of pure fatty ester oils has been that they tend to coalesce into large droplets rather than forming a continuous film, unless applied at high application rates. Adding surfactants can aid in dispersing of the methyl esters, but still results in incomplete surface coverage.
The use of oil-in-water emulsions is one method that looks promising for dispersing the oil phase as a continuous thin film. Oil-in-water emulsions with droplets that are small enough to produce kinetically stable emulsions will not cream to form a uniform surface film. Oil-in-water emulsions with large droplets will readily cream to form a continuous oil film, but will not be stable in dilute emulsions with a water external phase during storage of the product, making for a very limited shelf life.
By making a concentrated emulsion with 50% oil-in-water and adding a thickening agent to the water phase the emulsion can be made stable as a concentrate with good product shelf life, but when applied to the water surface the emulsion dilutes and breaks to form an effective oil film for mosquito control. The present invention provides oil-in-water emulsions of esters of fatty acid that are both stable prior to use in concentrated form and yet when applied to the water surface can break to form thin continuous film layers on the surface of water that are effective as larvicides.